Added New Option 'HTTPProxyOverrideReturnedErrors' which lets the server override

the error pages returned from the proxied server and replace them with the standard
server error handling on the main server.

Reviewed by:	Graham, Chuck


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@91092 13f79535-47bb-0310-9956-ffa450edef68